This invention relates to a frequency-voltage conversion circuit and a receiving apparatus applicable for a direct conversion receiver which receives and demodulates a FSK (Frequency Shift Keying) signal.
A superheterodyne method and a direct conversion method are generally used in a FSK (Frequency Shift Keying) receiver. In -each method, demodulation is carried out by the use of the known F-V (Frequency-Voltage) conversion.
Referring to FIG. 1, description will be made about a related direct conversion receiver using the F-V conversion.
In a Weber receiver illustrated in FIG. 1 the direct conversion receiver, a base-band cross signal is brought up to intermediate frequency (namely, up-conversion is conducted), and the F-V conversion is performed.
The FSK signal sent from a receiver (not shown) is received by an antenna 101, is amplified by a high frequency amplifier 102, and is given to mixers 103 and 104, respectively.
A local oscillator 107 produces an oscillation signal. The oscillation signal is shifted with .pi./2 by the use of a .pi./2 shifter 105, and is given to the mixer 103. Further, the frequency signal from the local oscillator 107 is directly given to the mixer 104.
Low pass filters (hereinafter, abbreviated as LPFs) 106 and 108 are connected to the mixers 103 and 104, respectively. In this condition, output signals from the mixers 103 and 104 are given to the LPFs 106 and 108, respectively.
Each of the LPFs 106 and 108 has passing band equivalent to the base band signal, and realizes or obtains selectivity between adjacent channels. Further, the LPFs 106 and 108 supply output signals corresponding to signals from the mixers 103 and 104 into an up-conversion portion 130.
In this case, the up-conversion portion 130 is composed of mixers 109 and 110, a local oscillator 113, a .pi./2 shifter, and an adder 112, as illustrated in FIG. 1.
With this structure, the mixer 109 is given with an oscillation signal from the local oscillator 113. Further, the oscillation signal from the local oscillator 113 is shifted with .pi./2 by a .pi./2 shifter 111, and is given to the mixer 110.
Signals multiplied by the mixers 109 and 110 are added by the adder 112. Alternatively, the multiplied signals may be subtracted by a subtracter (not shown). An output signal of the adder 112 is converted by the use of a delay detection portion 114.
In the above-mentioned Weber receiver 131, a carrier wave frequency of the received FSK signal is defined as .omega./2.pi. while frequency deviation is defined as .+-..DELTA..omega./2.pi.. In this condition, the received FSK signal Sr.sub.FSK is represented by the following equation. EQU Sr.sub.FSK =cos(.omega..+-..DELTA..omega.)t
In this event, when the output signal S.sub.OSC1 of the local oscillator 107 is defined as S.sub.OSC1 =sin .omega.t, the output signals SMOG and S.sub.MIX4 of the mixers 103 and 104 are represented by the following equations, respectively. EQU S.sub.MIX3 =cos(.omega..+-..DELTA..omega.)t.multidot.cos .omega.t=1/ 2{cos(.omega..+-..DELTA..omega.+.omega.)t+cos(.omega..+-..DELTA..omega.-.o mega.)t}=1/2{cos(2.omega..+-..DELTA..omega.)t+cos(.+-..DELTA..omega.t)} EQU S.sub.MIX4 =cos(.omega..+-..DELTA..omega.)t.multidot.sin .omega.t=1/2{(sin (.omega..+-..DELTA..omega.+.omega.)t+sin(.omega..+-..DELTA..omega.-.omega. )t}=1/2{sin (2.omega..+-..DELTA..omega.)t+sin(.+-..DELTA..omega.t)}
First terms of these equations are removed by the LPFs 106 and 108. Therefore, the outputs S.sub.LPF6 and S.sub.LPF8 of the LPFs 106 and 108 are represented by the following equations. EQU S.sub.LPF6 =1/2{cos(.DELTA..omega.t)} (1) EQU S.sub.LPF8 =.+-.1/2{sin(.DELTA..omega.t)} (2)
In this case, when calculation is carried out without limiter amplifiers 128 and 129 so as to be readily understood, an output signal Vout of the up-conversion portion 130 is modified as follows. Herein, it is to be noted that the output signal of the local oscillator 113 is defined by S.sub.OSC2 =sin .omega.2t. EQU Vout=1/2{cos(.DELTA..omega.t)sin .omega.2t)}.+-.1/2{sin (.DELTA..omega.t)cos .omega.2t)}=1/2{sin (.omega.2.+-..DELTA..omega.))}
From the above-mentioned result, the base band signal I, Q is converted to a signal having frequency deviation of .+-..DELTA..omega./2.pi. when the intermediate frequency .omega.2/2.pi. is defined as a center.
Subsequently, when the limiter amplifiers 128 and 129 are inserted between the LPF 106 and the mixer 109 or between the LPF 108 and the mixer 110, the condition is explained as follows.
When inputs into the mixers 109 and 110 becomes rectangular wave by the limiter amplifiers 128 and 129, outputs S.sub.LPT6' and S.sub.LPF8' are modified as follows by Fourier transforming the above-mentioned equations (1) and (2). Herein, it is to be noted that constant is defined as k=2/.pi.. EQU S.sub.LPF6' =k{cos(.DELTA..omega.t)}+1/3.multidot.cos(3.DELTA..omega.t)+1/ 5.multidot.cos(5.DELTA..omega.t)+ . . . } (1') EQU S.sub.LPF8' =k{sin(.omega.2.+-..omega.)t+1/ 3.multidot.sin(3(.omega.2.+-..DELTA..omega.)t+1/ 5.multidot.sin(5(.omega.2.+-..DELTA..omega.)t)+ . . . } (2')
Namely, the output Vout' of the up-conversion portion 130 is similarly considered to be the modification of the above-mentioned equation (3). Thereby, the following equation is introduced. EQU Vout=k{sin(.omega.2.+-..omega.)t+1/ 3.multidot.sin(3(.omega.2.+-..DELTA..omega.)t+1/ 5.multidot.sin(5(.omega.2.+-..DELTA..omega.)t)+ . . . } (3')
Consequently, it is found out that the conversion-up becomes possible even when the limiter amplifiers 128 and 129 are inserted between the LPF 106 and the mixer 109 or between the LPF 108 and the mixer 110.
Although the Weber receiver 131 has been suggested as a SSB (Single Side Band) receiver, it is found out that the Weber receiver 131 is applicable as the FSK receiver, as explained above.
The output signal of the adder 112 is given to the delay detection portion 114, and the F-V conversion is carried out in the delay detection portion 114.
In FIG. 2, a detail structure of the delay detection portion 114 is illustrated. Further, a timing chart showing change (waveform) of each signal of each portion in the delay detection portion 114 is illustrated in FIG. 3.
A signal V.sub.A from the adder 112 is converted into output signals V.sub.B and V.sub.C by removing amplitude demodulation components by the use of a limiter amplifier 119.
Subsequently, the output signals V.sub.B and V.sub.C are converted into signals V.sub.D and V.sub.E having desired slopes at rising through common-emitter transistors 121 and 221. Further, the signals VD and V.sub.E are converted into signals V.sub.F and V.sub.G by comparators 123 and 223 given with threshold level V.sub.TH26 from a reference voltage 126.
In this event, the transistors 121 and 221 are coupled to constant current sources 120, 220 and capacitors 122, 222, respectively.
Moreover, the signals V.sub.F and V.sub.G are converted into a signal V.sub.H via an AND gate (namely, logical product). Thereby, pulse signal line, which has constant amplitude and constant delay time .tau., is formed, as illustrated in FIG. 3.
Finally, the pulse signal line V.sub.H is integrated by a LPF 125, and converted into a voltage value V.sub.I corresponding to frequency. Further, the obtained voltage V.sub.I is converted into a logic data signal consisting of "1" and "0" by a converter (not shown).
In FIG. 4, frequency spectrums are illustrated so as to explain the above-mentioned structure. In an intermediate stage in the FIG. 4, center frequency between frequency of "1" and frequency of "0" becomes carrier wave frequency.
In FIG. 5, characteristic obtained the delay detection portion 114 is illustrated. In the above-mentioned example, demodulation sensitivity KD is defined as KD=2.tau.V [V/Hz]. Consequently, the characteristic is affected by variation of .tau. and V Herein, it is to be noted that .tau. represents delay time while V indicates output amplitude of the signal V.sub.H.
Moreover, the delay time .tau. is inversely proportional to variation of the constant current sources 120 and 220 illustrated in FIG. 2, and is proportional to variation of static capacitance of the capacitors 122 and 222. Further, the delay time .tau. is proportional to the threshold voltage V.sub.TH26.
Specifically, the demodulation sensitivity is fluctuated by variation of manufacturing condition. In addition, the F-V conversion output amplitude is varied in the direct-conversion method using the F-V conversion. As a result, receiving condition may be deteriorated.
Further, the power supply voltage is restricted from the same reason, and reneality of the F-V conversion is degraded. In consequence, receiving condition is also degraded.